Project Summary Healthy cognitive processing is associated with well-defined brain oscillations, which are not only an indication of accurate timing of neuronal activity within brain regions, but also of the coordination of neural activity patterns between brain regions. Consistent with this notion, some aspects of oscillatory activity in at least a subset of brain regions are disrupted in any of the major psychiatric and neurological diseases. However, evidence for a causal link between disrupted oscillations and impaired behavior has remained sparse. In particular, it has been challenging to manipulate timing within brain circuits without simultaneously disrupting other firing statistics of neuronal populations, such as their firing sparsity or firing rate. Preliminary data are presented which indicate that optogenetic rhythmic stimulation of medial septal PV neurons at frequencies that are higher than the endogenous theta frequency (? 10 Hz) alters hippocampal spike timing, but without changing other firing statistics, such as spatial firing patterns, firing rate, and theta phase precession. Furthermore, we found that stimulation at 8 Hz was without effect on memory performance while stimulation at ?10 Hz resulted in a memory impairment that was comparable in its severity to a complete hippocampal lesion. We therefore hypothesize that minor shifts in the timing of neuronal activity result in a loss of coordination between neuronal networks in the hippocampus, medial entorhinal cortex, and medial prefrontal cortex, such that these brain regions can no longer support spatial working memory. We will perform three aims to address this hypothesis. (1) Determine the minimal time shift at which memory deficits emerge and measure the coordination of hippocampal neural activity while stimulating at frequencies with and without memory deficits. (2) Determine whether manipulations of theta frequency critically alter firing patterns in medial entorhinal cortex (mEC). (3) Determine whether oscillations within the endogenous theta frequency range are necessary for the coordination between hippocampus and medial prefrontal cortex (mPFC). In each of the three aims, we will perform LFP and single unit recordings while mice perform a spatial alternation task. Two versions of the task will be compared, a delayed version which is dependent on hippocampal and prefrontal function, and a continuous version, which is included as a control. While mice are performing the task, the medial septal area will be stimulated at frequencies that do not result in memory deficits or stimulated at frequencies that result in memory deficits. These conditions will be compared to identify the critical changes in neural activity in hippocampus, mEC, and mPFC at the transition to the memory impairment. Taken together, the combined manipulations and recordings in the behavioral task will provide evidence for a causal relation between precise timing and memory function and for a role of oscillations in coordinating the exchange of information between brain regions.